CATALYTIC HYDROLYSIS OF DICHLORODIFLUOROMETHANE (CFC-12) ON SOL-GEL-DERIVED TITANIA UNMODIFIED AND MODIFIED WITH H2SO4

Catalytic hydrolysis of dichlorodifluoromethane (CFC-12) in a humid ai r stream was studied over pure and sulfate promoted TiO2 catalysts whi ch were prepared by sol-gel methods. The results showed that complete conversion of CFC-12 on unmodified TiO2 was achieved at reaction tempe ratures higher than 340 degrees C under the employed reaction conditio ns, The selectivity to CO2 ([CO2](produced)/[CFC-12](converted)) varie d from 0.5 to 0.88 over the range of 250-350 degrees C and CClF3 (CFC- 13) was detected as the main by-product. Surface fluorination of the T iO2 catalyst during the reaction improved its activity, induced the fo rmation of the fluorinated by-product CFC-13, and changed such propert ies of the catalyst as its specific surface area,pore size distributio n, and crystal size. It was found that the catalytic and structural pr operties of TiO2 were greatly improved by sulfation. The sulfated TiO2 (TiO2/SO42-) exhibited excellent reaction activity and selectivity fo r CFC-12 catalytic decomposition at low reaction temperatures (190-250 degrees C) while retaining a stable structure. Complete decomposition of CFC-12 with stoichiometric production of CO2 was observed over the TiO2/SO42- catalyst at 250 degrees C under otherwise identical reacti on conditions as used for pure TiO2. Results of catalyst characterizat ion by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) , Fourier transform infrared spectroscopy (FTIR), and N-2 sorption ana lysis indicated that surface sulfate species formed on the sulfated Ti O2. When compared to unmodified TiO2, the sulfated TiO2 exhibited high er resistance to crystal phase transformation from anatase to rutile, higher resistance to the deleterious effects of fluorination of the ca talyst, and higher specific surface area. Fluorination of the catalyst did not improve the activity of sulfated TiO2 and no CFC-13 was detec ted as a by-product, indicating that fluorine was not involved in the formation of reaction sites over the sulfated TiO2 catalysts. (C) 1997 Academic Press.